Piezoelectric pressure sensors are known and are widely used. Thus, they are used in pressure indexing of internal combustion engines to detect a cylinder pressure prevailing in a pressure chamber as a function of the crankshaft position or a time. Among the different types of internal combustion engines are four-stroke engines and two-stroke engines such as gasoline engines, diesel engines, Wankel engines, etc. In marine diesel engines, piezoelectric pressure sensors are used for long-term monitoring of a cylinder pressure. Piezoelectric pressure sensors are used to monitor fast pressure profiles that usually are in the range of 150 to 250 bar but including pressure peaks of 500 bar and higher if pre-ignition and engine knocking occur. However, piezoelectric pressure sensors also can be used in pressure monitoring in jet engines, gas turbines, steam turbines, steam engines, etc.
U.S. Pat. No. 3,364,368, which is hereby incorporated herein by this reference for all purposes, discloses a piezoelectric pressure sensor that includes a membrane that protrudes directly into the pressure chamber through a bore of the pressure chamber. An edge of the membrane is welded to a housing of the piezoelectric pressure sensor. The pressure profile captured by the membrane acts onto a piezoelectric sensor, which is arranged within the housing in the proximity of the membrane. The pressure profile generates electric polarization charges on the piezoelectric sensor, and these charges are transmitted as signals via an electrode. The magnitudes of the signals are proportional to the magnitudes of the pressure profiles. The electrode is arranged on the piezoelectric sensor. By means of an electrical conductor, the signals are transmitted from the electrode to a socket for a plug connection of a signal cable to an evaluation unit. The socket is arranged on a side of the housing that faces away from the membrane.
The piezoelectric sensor is mechanically pre-stressed via a pre-stressing sleeve. With respect to construction, this is achieved by a circular-symmetrical construction of the piezoelectric pressure sensor. The housing is cylindrical in shape. The membrane is frontally attached to the housing, and to the rear, the piezoelectric sensor is arranged along a longitudinal axis of the housing. The electrical conductor extends centrally through the piezoelectric sensor and the housing. In the direction of the longitudinal axis, the pre-stressing sleeve is arranged radially outward of the piezoelectric sensor. A rear end of the pre-stressing sleeve is welded to the housing, while a front end of the pre-stressing sleeve is welded to the membrane. Thus, the pre-stressing assembly seals the piezoelectric sensor against the socket in a vacuum-tight and pressure-tight manner.
However, during use of the piezoelectric pressure sensor, the plug connection between the socket and the signal cable is permanently exposed to strong engine vibrations and high temperatures of 200° C. and above, and this exposure may lead to micro friction and fretting corrosion of the electrical plug contacts. In addition, outgassing of the signal cable sheath may occur at high temperatures, leading to friction polymerization at the electrical plug connection. Furthermore, diffusion of base metals to a contact surface of the electrical plug contacts may occur in the plug connection at high temperatures, leading to build-up of an oxide layer that is present in this area. These effects can occur separately or in combination. As a result, the electrical contact resistance of the electrical plug contacts may change. Thus, the electrical contact resistance may increase from the mΩ range by several orders of magnitude into the MO range and distort the signals transmitted to the evaluation unit, resulting in incorrect signal evaluations.
It is a first object of the invention to provide a piezoelectric pressure sensor wherein signal distortion in the signal output is effectively prevented. It is another object of the present invention to provide a piezoelectric pressure sensor wherein the signal output additionally is mechanically stable. Another object of the invention is to provide a process for cost-effective manufacturing of a piezoelectric pressure sensor. These and other objects desirably are achieved by the features described more fully below.